Measurement of spin-orbit torques from Rashba and topological surface states

Resumen del trabajo presentado al 1st Workshop Spain-Taiwan: "2D Materials and Interfaces for Spintronics", celebrado en Barcelona (España) del 23 al 25 de octubre de 2017.-- et al.The Dirac bands formed at the interface between a three-dimensional topological insulator (TI) and a trivial insulator are characterized by their spin-helicity, arising from the combination of a large spin-orbit coupling and time reversal invariance. Spin-momentum locking potentially makes TIs extremely efficient charge-to-spin converters, able to exert a large spin-orbit torque (SOT) on an adjacent ferromagnet (FM). SOT-induced ferromagnetic resonance (SOT-FMR) gives access to the nature and magnitude of the torques. So far, only few studies have focused on SOT-FMR in TI/FM multilayers, and large spin torques have been reported. However, little is known about TI/metal interfaces. Charge doping together with a strong hybridization between topological bands and metallic states is suspected to deeply weaken the spin-momentum locking, which questions the origin of the large SOT measured. We will present measurements of the non-equilibrium SOTs by current-induced ferromagnetic resonance on TI/NM/FM multilayers embedded in coplanar waveguides. A systematic combination of spin transport and photoemission shows that the chemical and electronic properties of the TI/Py interface can be tuned by using various normal metal spacers (NM=Ag,Al,Te), which results in a strong modulation of the SOTs. Details about the fabrication of single-crystal TI films by molecular beam epitaxy and their characterization will also be presented. We will show that crystal twin defects can be fully suppressed by using lattice-matched substrates and proper growth conditions, which is associated with a low doping of the free surface. Band bending effects at interfaces and their compensation by alloying will also be addressed.Peer reviewe